1. Field of the Invention
This invention relates to a process for the production of rutile mixed-phase pigments by calcination of solid mixtures of titanium dioxide and/or titanium dioxide hydrate with dye-producing metal compounds, particularly of chromium and/or nickel, optionally with addition of compounds of antimony, niobium or tungsten, in an indirectly heated rotary kiln to which air is supplied.
2. Background Information
Rutile mixed-phase pigments are known from DE-A-12 87 236. These known pigments are colored pigments which are obtained by incorporation of colored transition elements in the crystal lattice of the rutile. Metals such as these are predominantly incorporated as guest component in the host lattice of the rutile, of which the cation radii are of the order of the radius of the titanium(IV) ion. If the valency of the cation to be incorporated differs from that of the titanium ion, namely four, another cation having a different valency is added for statistical valency equalization.
Nickel and chromium rutile mixed-phase pigments have acquired considerable significant for large-scale production. In their case, oxides of relatively high valency, particularly antimony oxide and less frequently niobium or tungsten oxide, are incorporated for valency equalization with the chromophoric nickel and chromium nickel (Ullmanns Encyklopadie der technischen Chemie, Verlag Chemie GmbH, Wienheim 1979, 4th Edition, Vol. 18, page 608).
The mixed phase pigments with a rutile lattice are produced by calcination of homogeneous mixtures of titanium dioxide as the host component and the chromophoric guest components at temperatures of up to 1200.degree. C. Instead of oxidic guest components, heat-sensitive compounds of the metals on which the components are based may also be mixed with titanium dioxide, titanium hydroxide or titanium dioxide hydrate, changing on heating in air into the corresponding oxides, for example, hydrates, hydroxides, carbonates, acetate, nitrates or formates.
The raw material mixtures are usually calcined in rotary kilns which may be either directly or indirectly heated. Where the rotary kilns are directly heated, the starting material is best an aqueous suspension containing the starting components, which largely avoids solids being discharged from the kiln with the smoke gases. Where dry mixtures of the starting components are used, it is better to use indirectly heated kilns because, in their case, the quantity of waste gas can be more or less considerably reduced, depending on the starting materials, so that there is no danger of a high discharge of solids.
DE-A-30 19 172 describes one such process for the production of rutile mixed-phase pigments. The advantage of this process over earlier processes is said to lie in the economy of the fully continuous rotary-kiln process for the calcination step.
To achieve the advantages of the process, a relatively long rotary kiln with a length-to-diameter ratio of 20:1 has to be installed. Apart from the fact that relatively long indirectly heated rotary kilns, which are heated to temperatures of up to about 1200.degree. C., as measured in the product to be calcined, are fairly difficult to construct and operate, they do of course show greater irradiation losses than shorter kilns of the same capacity.
In another embodiment of the process, air is supplied to the kiln in co-current with the material to be calcined, because countercurrent operation, which is of greater advantage in terms of energy, gives products of no real value.
It is already known that titanium dioxide hydrate, also known as titanium oxide hydroxide or metatitanic acid, can be used in the production of rutile mixed phases (DE-A26 05 651).
The suspension obtained from the hydrolysis of titanium or digestion solutions in sulfuric acid after filtration and washing, with solids contents of around 305 by weight, is used in this process. The titanium dioxide hydrate in this aqueous suspension, which usually accumulates in the form of a paste or a filter cake, has a low pH value of around 1 to 2 and still contains considerable quantities of bound sulfuric acid of the order of 5 to 10% by weight which cannot be washed out (Ullmanns Encyklopadie der technischen Chemie, Vol. 18, 1979, page 576, column 1).
The disadvantage of this titanium-containing material as a starting material for the rutile mixed-phase pigments is that the sulfuric acid is released only on calcination. However, this creates an increased risk of corrosion in the rotary kiln. In addition, considerable expense is involved in treating the waste gases.
In addition, the use of these titanium compounds does not lead to improved rutile mixed-phase pigments. The use of calcined anatase is also known from DE-A-11 36 041, but does not lead to the required pigment quality.